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L 30 Electricity and Magnetism [7]


L 30 Electricity and Magnetism [7] Electromagnetic Waves Faraday laid the groundwork with his discovery of electromagnetic induction Maxwell added the last piece of ... – PowerPoint PPT presentation

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Title: L 30 Electricity and Magnetism [7]

L 30 Electricity and Magnetism 7
  • Electromagnetic Waves
  • Faraday laid the groundwork with his discovery of
    electromagnetic induction
  • Maxwell added the last piece of the puzzle
  • Hertz made the experimental discovery

James Clerk Maxwell
  • Faraday showed that a changing magnetic field can
    generate a current.
  • Another way to look at this is to say that a
    changing magnetic field can create an electric
  • Maxwell argued that a changing electric field
    should then also create a magnetic field.

Electromagnetic waves
  • A wave is a disturbance that propagates in a
  • transverse waves on a string
  • longitudinal sound waves in air
  • an electromagnetic wave is an electric and
    magnetic disturbance that propagates through
    space (even vacuum) at the speed of light
    299,792,458 m/s or 186,000 miles per second.
  • EM waves include radio, microwaves, x-rays, light
    waves, gamma rays

Electric and Magnetic Fields
  • electric charges produce electric fields
  • electric currents (moving charges) produce
    magnetic fields
  • an electromagnetic wave is a pattern of electric
    and magnetic fields that vibrate together in
    space and time in a synchronous fashion

Electric Field Magnetic Field
electric field of a positive charge
magnetic field of a current in a wire
the generation of an electromagnetic wave
electric field
magnetic field
The time varying electric field generated the
time varying magnetic field which generates the
time varying electric field and so on and so on
. . . .
EM waves transverse
  • the electromagnetic wave is a transverse wave,
    the electric and magnetic fields oscillate in the
    direction perpendicular to the direction of

E field
direction of propagation
B field
Electromagnetic waves
  • the EM wave propagates because the electric field
    recreates the magnetic field and the magnetic
    field recreates the electric field
  • an oscillating voltage applied to the antenna
    makes the charges in the antenna vibrate up and
    down sending out a synchronized pattern of
    electric and magnetic fields
  • an electromagnetic wave must have both an
    electric and magnetic field component

How radio waves are produced
transmission line
High Frequency Oscillator
Dipole Antenna
Radio antenna
the oscillating electric field of the EM wave
causes the electrons in the receiving antenna to
oscillate at the same frequency
the amplifier converts the electrical signal to
sound waves
Common frequency band AM radio - 535 KHZ to 1.7
MHZ Short wave radio - bands from 5.9 to 26.1
MHZ Citizens band (CB) radio - 26.96 to 27.41
MHZ Television stations - 54 to 88 MHZ for
channels 2 through
6 FM radio - 88 to 108 MHZ Television stations -
174 to 220 MHZ for channels 7 through 13
1 KHZ 1000 Hz, 1 MHZ 1,000,000 Hz
Frequency Bands
Other common bands
Garage door openers, alarm systems, etc. -
Around 40 megahertz Standard cordless phones
Bands from 40 to 50 megahertz Baby monitors 49
megahertz Radio controlled airplanes Around 72
megahertz, which is different from... Radio
controlled cars Around 75 megahertz Wildlife
tracking collars 215 to 220 megahertz MIR space
station 145 megahertz and 437 megahertz Cell
phones 824 to 849 megahertz New 900-MHz
cordless phones Obviously around 900 megahertz!
Air traffic control radar 960 to 1,215
megahertz Global Positioning System 1,227 and
1,575 megahertz Deep space radio communications
2290 megahertz to 2300 megahertz
What is Bandwidth?
  • the term bandwidth has two common meanings that
    are related
  • range within a band of frequencies, e.g. the
    bandwidth between 40.1 MHZ and 40.2 MHZ is 0.1
  • the amount of data that can be transmitted in a
    fixed amount of time measured in bits per
    second or bps.

the bandwidth problem
  • the FCC allocates bandwidth for commercial
  • the problem is how many stations can broadcast
    within a given band
  • each station is allotted a frequency, but the
    output of one station may overlap a bit with
    another station
  • better technologies allow more channels within a
    band without interference

Output Power
The golden rule
  • the golden rule for waves speed wavelength
    times frequency applies to electromagnetic waves
  • the speed is roughly 300,000,000 m/s
  • for example, the wavelength of a 1 MHZ radio wave
    is wavelength speed/frequency
  • 300 meters

Transmitting information
  • a signal like the one above does not transmit any
    information it just goes up and down, up and
  • both the amplitude (A) and the period (T) or
    frequency f 1 / T never change

Amplitude Modulation (AM)
  • with AM the amplitude of the wave signal is
    modulated (changed).
  • the information is coded into the way that the
    amplitude is modulated

Frequency modulation (FM)
  • with FM signals the frequency of the signal is
  • information is coded into the way that the
    modulation frequency is varied

  • are in the frequency range of a few billion Hz or
    wavelengths of about several cm (about the same
    range as radar ? the Radarange
  • How do microwaves heat water?
  • Remember that the water molecule has a positive
    end and a negative end.
  • The electric field of the microwave grabs onto
    these charges and shakes them violently a few
    billion times each second
  • all this shaking energizes the molecules making
    the water hotter and hotter.

the metal screen on the door allows you to see in
but does not let the microwaves out
No metal in the microwave!
  • if you have ever accidentally left a fork in the
    microwave you know that you get a spectacular
    array of arcs inside.
  • The microwaves can cause charges to build up on
    the sharp edges of the fork
  • If enough charge builds up, an arc can occur
  • The metal walls of the microwave are smooth and
    act to reflect the microwaves back into the food
    where they belong!

  • x-rays are very short wavelength electromagnetic
  • how short? 0.00000001 m
  • by contrast, a 100 MHZ radio wave has a
    wavelength of 3 meters
  • x-rays and radio waves are both electromagnetic
    waves that differ only in wavelength and frequency

How are x-rays produced?
x-ray tube
  • when electrons that have been accelerated
  • through about 50,000 volts slam into a piece
  • of copper, some of the electron energy is
  • converted to x-rays
  • x-rays are energetic enough to penetrate
  • through soft tissue
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